Certain precision equipments, such as the electromechanical Winchester Hard Disk Unit (HDU), are not hermetically sealed and must be provided with breather ports to equalize internal and external (ambient) pressures. Without the equalization of pressures, air leaks may occur through various component seals. Such leaks would permit dust to enter the HDU, thereby causing possible electrical and mechanical malfunctions.
Pressure differentials commonly result from changes in temperature and altitude. As to the former, when an HDU is cold and placed in a warm, ambient environment, internal air is expelled through the breather ports as the unit reaches operating temperature. This condition poses little or no operational problems. However, when a warm HDU is placed in a cold environment, as the unit cools down, ambient air is drawn into the unit through its breather ports. This is the condition which exists when a unit operating at its maximum temperature is turned off and allowed to cool in a cold environment. If it is assumed that the HDU has been operating in high relative humidity greater than its design limit of 80%, then during cool down, moisture enters the unit through the ports and coats the read/write heads and magnetic disk media which are in a "park" position. In this case, the heads and disk media tend to stick to each other inhibiting the restart of the unit. Actually, many attempts at starting may be required before the bond between heads and media is broken. Adverse side effects from the last mentioned procedure are common ----the media surface may be damaged and the media particles which have been broken free in the start attempts constitute a source of further damage to the unit.
Several factors complicate the elimination of the foregoing moisture problem in the HDU. Normally, HDU's are mounted in very compact module enclosures, which preclude the use of bellows over the breather ports ----the bellows would have permitted internal air to expand and contract without causing the unit to ingest any of the ambient air. Also, the HDU transmits heat to its surrounding air through its housing. Therefore, its exterior surfaces must be free of structures which might impede such heat transfer and additionally, might restrict the free movement of heat-removing air surrounding the unit as provided with the module enclosure. Finally, since the HDU assembly is generally an off-the-shelf, finished product, no auxiliary electrical power terminals or mounting holes are available in the HDU housing to accept a dry air breather device. Therefore the mounting of such a device on the unit is a major consideration.
What is desired is a low-profile, small static type device that doesn't require electrical power and can readily be mounted on, and subsequently removed for service from, the HDU external housing. Such considerations obviate the use of mechanical type devices which require electrical power or high velocity vapor in order to remove water by impaction or centrifugal force. The present invention meets the above-mentioned requirements in an air breather assembly comprised of a unique enclosure, readily mountable on the HDU housing, and providing for the efficient and effective use of a specific type of desiccant disposed in the enclosure.